C1q and mannose binding lectin engagement of cell surface calreticulin andCD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostas is, organogenesis, remodeling, development, and maintenance of the immune s ystem, protection against neoplasia, and resolution of inflammation. The me chanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we p rovide evidence that Clq and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingesti on of these by ligation on the phagocyte surface of the multifunctional pro tein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha -2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune sy stem. Ingestion of the apoptotic cells through calreticulin/CD91 stimulatio n is further shown to involve the process of macropinocytosis, implicated a s a primitive and relatively nonselective uptake mechanism for C1q- and MBL -enhanced engulfment of whole, intact apoptotic cells, as well as cell debr is and foreign organisms to which these molecules may bind.